October 25, 2013…The US. Department of Energy (DOE) has published a new report, Color Maintenance of LEDs in Laboratory and Field Applications. The DOE explained in a statement, that although concern for failure of LED lighting products has largely been focused on lumen maintenance, color shift is a cause of early failure for some products, especially in applications where visual appearance is critical. The report notes that color shift can cause not only a change in color of the emitted light, it can also potentially change the appearance and color of objects being illuminated.
The report discusses field data from DOE’s GATEWAY program. The Gateway program found that many LED lamps installed in museums have changed color beyond a reasonable tolerance well before their rated lifetimes were reached.
Furthermore, the DOE says that laboratory data from its CALiPER program shows that many early LED products shifted beyond acceptable tolerances in as short a as a few thousand hours. But CALiPER results also reveal that testing of the L Prize®-winning lamp has shown that commercially available LED products can have exemplary color stability unmatched by traditional light sources.
The report outlines the metrics used for communicating color shift, and explains to end users how to monitor chromaticity and what to look for in manufacturer warranties. Also covered are the physical changes that have been shown to lead to color shift in some types of LED packages. As with lamps and luminaires, the data for LED packages reveals a wide variety of products are available. The DOE says that more detailed and standardized information is necessary in order for specifiers and consumers to make educated choices.
The report defines color stability as a lamp’s ability to maintain a spectral power distribution over time. Notably, this is separate from color consistency, which is the initial lamp-to-lamp variation in spectral power distribution. According to the report, changes in spectral output resulting from changes to the ambient environment are also a distinct issue, as is depreciation in radiant flux. The DOE indicates that these issues are interrelated.
Chromaticity coordinates, a basic principal of the CIE system of colorimetry [CIE 2004] or a metric derived from them, is often used to describe the color of a light source. The chromaticity coordinates of a source provide a numerical representation of the color of the light, but offer little indication of how the source will render specific object colors. Hence, color shift is most frequently reported as the difference or change in coordinates.
The DOE examined many different lamps installed for the Smithsonian Gateway Project. Gateway program also collected and measured lamps from the Hallie Ford Museum of Art at Willamette University, an installation that was not part of a Gateway demonstration.
The Energy Star® program does mandate that the change in chromaticity coordinates expressed as Δu’v’ at 6,000 hours of operation not exceed 0.007. The DOE says that while this standard is a reasonable starting point, it may not be strict enough to ensure very high-quality lighting, especially because LED product lifetimes routinely far exceed 6,000 hours. Change of 0.001 is equivalent to a one-step MacAdam Ellipse.
The report pointed out that numerous contributing factors can effect color shift, and it is therefore especially complicated at the complete product level. As a result, in applications where color quality is important, some products have useful lifetimes that are substantially shorter than their rated lifetime. This may in turn affect the anticipated payback. The report states that concerns could be alieviated with manufacturer warranties, but few currently available warranties cover color shift.
In the report’s conclusion, the DOE advocates: Continued development of LED package technology to improve color stability; Development of standards for predicting long-term color shift performance from a more limited set of measured data; Investigation of the interaction of multiple factors affecting the color stability of complete LED lamps and luminaires; Education within all segments of the lighting industry regarding the issue of color shift and the tools to communicate it properly; and more widespread inclusion of color shift in product warranties, pursuant to an agreed-upon method for documenting color shift beyond an established threshold.